As a display element provided in a display device, there have hitherto been an electro-optic element whose luminance is controlled by an applied voltage, and an electro-optic element whose luminance is controlled by a flowing current. Examples of the electro-optic element whose luminance is controlled by an applied voltage include a liquid crystal display element. Meanwhile, examples of the electro-optic element whose luminance is controlled by a flowing current include an organic EL element. The organic EL display device using the organic EL element as a self-emitting element can be reduced in thickness and power consumption and increased in luminance as compared to the liquid crystal display device which generally requires a backlight, a color filter and the like, and is thus under active development in recent years.
There are two kinds of drive systems for the organic EL display device, which are a passive matrix system (also called simple matrix system) and an active matrix system. An organic EL display device that employs the passive matrix system (hereinafter referred to as “passive matrix-type organic EL display device”) has a simple structure, but is difficult to increase in size and resolution. In contrast, an organic EL display device that employs the active matrix system (hereinafter referred to as “active matrix-type organic EL display device”) can easily achieve a large size and high resolution as compared to the passive matrix-type organic EL display device.
In the active matrix-type organic EL display device, there is used a drive transistor which controls a drive current to be supplied to the organic EL element in accordance with a voltage applied between a gate and a source. As the drive transistor, a thin-film transistor (which may be hereinafter abbreviated as “TFT”) is typically used. Examples of the thin-film transistor include an amorphous silicon TFT, an LIPS (Low Temperature Poly Silicon)-TFT, and an oxide TFT (TFT whose channel layer is formed of an oxide semiconductor). Further, examples of the oxide TFT include an IGZO-TFT whose channel layer is formed of InGaZnOx (hereinafter referred to as “IGZO”) as an oxide semiconductor mainly composed of indium (In), gallium (Ga), zinc (Zn) and oxygen (O). When the thin-film transistor is used as the drive transistor, there occur a variety of variations such as variations in threshold voltage, mobility, channel width, channel length, and thickness of a gate insulating film (gate insulating film capacitance) among pixel circuits. Since the characteristics of the drive transistor vary among pixel circuits, the drive currents vary. This results in deterioration in display quality. It is to be noted that variations in mobility, gate width, gate length and thickness of a gate insulating film may be hereinafter referred to collectively as a “variation in gain”.
There has hitherto been known an organic EL display device which performs a variety of compensation by use of a transistor and/or a capacitance or the like in the pixel circuit. Patent Document 1 discloses an organic EL display device, where a transistor for detecting a fluctuation of a threshold voltage of a drive transistor is provided in a pixel circuit, thereby compensating a variation in threshold voltage. Further, Patent Document 2 discloses an organic EL display device which compensates variations in both threshold voltage and mobility by use of a power supply line in which a low potential and a high potential are switched at predetermined timing and a parasitic capacitance formed at each end of an organic EL element. It is to be noted that in the present specification, compensation of the variation in threshold voltage and compensation of the variation in mobility (or gain) may be referred to as “threshold voltage compensation” and “mobility (or gain) compensation”, respectively. Further, Patent Document 3 discloses an organic EL display device which performs the threshold voltage compensation by use of a parasitic capacitance formed at each end of an organic EL element, and the like. Moreover, Non-Patent Document 1 discloses a pixel circuit, where two capacitance elements connected in series are provided between a gate and a source of a drive transistor, a voltage at one end of one of the capacitance elements is set to a value in accordance with a threshold voltage, and then the one capacitance element is connected between the gate and the source of the drive transistor while the other capacitance element is short-circuited, thereby performing the threshold voltage compensation.
Furthermore, there is known an organic EL display device which performs a variety of compensation by use of a circuit provided outside a pixel circuit (hereinafter simply referred to as “external circuit”). Patent Document 4 discloses an organic EL display device which compares a target current and a measured current, obtained by measuring a drive current made to flow by a drive transistor in an external circuit provided with respect to each column, and performs offset correction for performing the threshold voltage compensation on video data that is to be sent to a data driver in accordance with the comparison result. It should be noted that a configuration related to the external circuit disclosed in Patent Document 4 is also disclosed in Patent Document 5 (however, since a description of a compensation operation by the external circuit is more simplified in Patent Document 5 than in Patent Document 4, a description of Patent Document 5 will be hereinafter omitted). Further, Patent Document 6 discloses an organic EL display device which measures an average drive current of all pixel circuits in an external circuit to update a correcting gain and a correcting offset, and corrects image data by use of these correcting gain and correcting offset. It is thus possible to perform both the threshold voltage compensation and the gain compensation. Moreover, Patent Document 7 discloses an organic EL display device which measures a drive current of each pixel circuit to update a correcting gain and a correcting offset, and corrects image data by use of these correcting gain and correcting offset. It is possible to perform both the threshold voltage compensation and the gain compensation. Furthermore, Patent Document 8 discloses an organic EL display device which acquires a detection voltage Vsen in accordance with a threshold voltage by use of a current source in an external circuit, and controls a drive transistor with a voltage, obtained by adding a data fluctuation amount ΔVdata′ in accordance with a movement deviation amount MV corresponding to each pixel circuit to the detection voltage Vsen, thereby performing both the threshold voltage compensation and the mobility compensation. It is to be noted that the movement deviation amount MV corresponding to each pixel circuit is previously prepared in a memory.